34 BULLETIN 57, UNITED STATES NATIONAL MUSEUM. 



premolars together. In the genera with large, triangular- crowned 

 trenchant premolars (most Phylldstomidae, Natalidse, etc.) the inner 

 posterior edge of the upper canine has become knife-like. It is 

 crossed by the anterior edge of the anterior lower premolar with a 

 true shearing motion, while its cingulum is so formed as to fit the 

 point and posterior edge of the smaller tooth. 



Premolars. — In bats with terete premolars the upper and lower 

 teeth do not come actually in contact, those'of the mandible lying 

 distinctly nearer to the sagittal plane than those of the upper 

 jaw. The point of pm 2 comes slightly below and behind that of 

 pm 3 , while that of pm i approaches pm 3 and the cingulum of pm 2 

 The large upper premolar {pm 4 ) shears with its interior inner sur- 

 face close to but not in contact with the posterior outer surface of 

 pm „ while its main cusp and posterior cutting edge play against 

 the cusp and anterior outer surface of prd 1 °, the extreme point of 

 which nearly touches surface of heel. When the premolars are 

 trenchant the anterior edge of' each lower tooth shears against the 

 posterior inner edge of the preceding upper tooth, while the posterior 

 edge cuts upon the anterior edge of the corresponding tooth in the 

 upper jaw. The relations of the first lower premolar to the upper 

 canine have already been explained. 



Molars. — As the distance between the two row's of mandibular 

 teeth is sensibly less than that between the opposed sets, 6 it follows 

 that to obtain a uniformly shearing action of the molars the jaws 

 must be thrown either to the right or left before the beginning of 

 the stroke. When the mandible is at its extreme lateral position 

 the outer and inner cusps of the lower teeth are closely opposed, 

 respectively, to the styles and highest cusps of the upper teeth — that 

 is, "in the outer rows; prcd 1 to ps 1, hcd 1 to ms 1, prcd 2 to mts 1 

 and ps 2, hcd 2 to ms 2, prcd 3 to mts 2 and ps 3, hcd 3 to ms 3, in 

 the inner rows, pcd 1 to the apex of pm*, mcd 1 to pc 1, ecd 1 and 

 pcd 2 to mc 1, mcd 2 to pc 2, ecd 2 and pcd 3 to mc 2, mcd 3 to pc 3, 

 ecd 3 to mc 3. The triangles of the lower teeth with their concave 

 blade-like edges (commissures) tilted slightly inward, and termi- 

 nated externally by the recurved cusps face the spaces between the 

 similarly concave but outward-tilted edges of the upper triangles, 

 each lower triangle lying in front of the corresponding triangle of 



o For the sake of brevity the cusps of the three molars may be referred to by 

 their abbreviation followed by the numbers 1, 2, and 3. Thus prd l=proto- 

 conid of first lower molar ; mc 3=metacone of third upper molar. 



» In a specimen of Noctilio (No. 37435, Mona Island, West Indies), the 

 greatest distance between outer edges of upper toothrows is 12.2 mm. ; between 

 outer \edges of lower toothrows, 8.9 mm. In Myotis californicus (No. 25826, 

 San Diego County, California) the same measurements are, respectively, 

 4.9 mm. and 2.8 mm. 



